Add -p option to download command.

Allow the ability to download a subset of containers (--all with -p) or
a subset of objects within a container (container name with -p).

This patch also includes a drive-by fix for "download --all" which would
not actually download any objects (for me, at least) because the object
queue got filled with "stop" messages before the container workers had
run long enough to put work in the object queue.  Doh!

I also closed up a few holes where an (unexpected, obviously) Exception
could cause the process to hang because non-daemon threads still
existed.

Change-Id: I71c6935c60282b5353badc2dfce8a935d47e3bb7
Merge "Add -p option to download command."